Interesting concept, I enjoyed it but I felt like the bolder took to long to be destroyed and lost a few times before I was able to actually stop it. I thought the tutorial was nice and informative and the graphics looked nice. I like the detail of the green goo all over the bolder if you loose the game.
If you wanted to improve on it, some upgrades would be nice depending upon how quickly you get the bolder destroyed. You could turn it into a mobile game too pretty easily with the simplistic controls.
Overall fun game, please check out mine if you get a chance!